Molten metal will chemically react and alloy with almost any material it comes in contact with. As an example, molten aluminum exhibits extreme chemical reactivity when melted and will alloy and dissolve with most other metals such as steel and copper even though they have a much higher melting point. Typically, mold temperatures are kept to a minimum so that the molten metal cools before any substantial chemical reaction occurs.
It is known to apply mold coatings to a mold to prevent adhesion of the part to the mold. This is typically accomplished by lubricating the mold with a coating that is non-reactive with the metal and one that will provide a slippage layer, void of the metal, so that the metal part will release from the mold. Coatings are typically comprised of a multitude of fine particles which are applied to the mold suspended in a liquid, such as water or alcohol. Coatings for low pressure metal castings typically act to stop the metal from passing through the particles of the coating because the metal does not wet with the ceramic particles and there is not sufficient pressure to infiltrate the particles. As an example, boron nitride spray is often used as a release coating for aluminum and graphite spray is typically used for copper castings because of its low solubility.
Unfortunately, lubricant spray coatings do not work well when the mold is subject to high temperature and pressures. For example, in Pressure Infiltration Casting.TM. (PIC) as disclosed in U.S. Pat. No. 5,111,871, molds must be used at temperatures near or above the melting point of the metal to be cast so the mold will not cause the metal to solidify when it contacts the mold. This is accomplished by maintaining the mold within a heated pressure vessel. In order to completely fill the cavities in the mold and/or infiltrate any preforms of reinforcement material disposed in the mold, high pressures must be used. The high temperatures, in excess of 400.degree. C., and pressures, in excess of 50 PSI, cause the metal to be very reactive with mold surfaces and thus adherence of the metal to the mold is a major problem which, as yet, has not being satisfactorily overcome.
The present invention provides a coating for a mold which is specifically adapted to react with the outer surface of the molten material to exhaust its reactive potential before it can react the mold. In this manner, the part is prevented from adhering to the mold.